The present invention relates to a sample clock generator for an optical tomographic imaging apparatus suitably applied in an optical coherence tomography using an optical frequency-swept laser, and an optical tomographic imaging apparatus.
An optical coherence tomography (hereinafter referred to as “OCT”) is designed to measure a tomographic image of a living body using optical interferometry. Particularly, the OCT is widely used to acquire a two- or three-dimensional tomographic image of a cornea, a retina, and so on in ophthalmology. Several basic systems exist for such an OCT, and a technique called swept-source OCT, hereinafter referred to as “SS-OCT”, is receiving attention above all.
In the SS-OCT, a light in which an optical frequency has been swept at high speed is emitted from a light source, and the light is radiated on an object to be measured, such as an eyeball. A scattered light from the object to be measured is detected by means of an interferometer. A detected raw signal (interference signal) is an interference spectrum, and by Fourier-analyzing the interference signal, a scattered light intensity distribution can be obtained that is resolved in a depth direction of the object to be measured. Such scattered light intensity distribution resolved in the depth direction of the object to be measured is generally called an A-scan OCT signal. Further, by laterally scanning a beam radiated on the object to be measured, a two-dimensional scattered light intensity distribution or a three-dimensional scattered light intensity distribution can be obtained.
Generally, in the SS-OCT, sampling of the interference signal is performed at high speed. It is preferred that such sampling is performed at equal optical frequency intervals. For example, it is known that, if the sampling is not performed at equal optical frequency intervals (at even frequency intervals with respect to the optical frequency), a distortion is generated in the A-scan OCT signal to thereby deteriorate the resolution significantly.
As a method for performing a sampling at equal optical frequency intervals, a method in which an optically-generated sample clock is used is known. In this method, aside from the interferometer used in the OCT, a Mach-Zehnder interferometer is structured, for example (see Japanese Unexamined Patent Application Publication No. 2013-181790 and Published Japanese Patent No. 5269809, for example). An output of the Mach-Zehnder interferometer is a sine wave at equal intervals with respect to the frequency of an input light. Therefore, the output of the Mach-Zehnder interferometer (an interference signal for clock) can be used as the above-described sample clock. Such a sample clock is often called a k-clock.